leetcode No367. Valid Perfect Square

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Question

Given a positive integer num, write a function which returns True if num is a perfect square else False.

Note: Do not use any built-in library function such as sqrt.

Example 1:

Input: 16
Returns: True

Example 2:

Input: 14
Returns: False

Algorithm

法1：二分查找
法2：牛顿迭代法

Code

二分查找：

class Solution {
public:
    bool isPerfectSquare(int num) {
        long long left=0,right=num/2+1;
        while(left <= right){
            long long mid = left+(right-left)/2;
            if(mid*mid == num){
                return true;
            }
            else if(mid*mid < num){
                left = mid+1;
            }
            else{
                right = mid-1;
            }
        }
        return false;
    }
};

牛顿迭代法：

class Solution {
public:
    bool isPerfectSquare(int num) {
        long r = num;
        while (r*r > num)
            r = (r + num/r) / 2;
        return r*r == num;
    }
};